Thermionic cathode

ABSTRACT

A cathode is disclosed utilizing an emission material of barium oxide, calcium oxide and an oxide selected from the group consisting of cobalt oxide, manganese oxide and molybdenum oxide.

United States Patent [1 1 Cronin [1 1 3,760,218 1 Sept. 18,1973

[ THERMIONIC CATHODE [75] Inventor: Leo J. Cronin, Watsonville, Calif.

[73] Assignee: Spectra-Mat, lnc., Watsonville,

Calif.

22 Filed: Apr. 10,1972

21 Appl.No.:242,345

[52] U.S. Cl 313/346 R [51] Int. Cl 1-l0lj 1/14, HOlj 19/06 [58] Field of Search 313/346 R, 346 DC [56] References Cited UNITED STATES PATENTS 3,076,916 2/1963 Koppins 313/346 R 3,201,639 8/1965 Levi 313/346 R 3,458,749 7/1969 Van Stratum et al.. 313/346 R 3,569,769 3/1971 Takanashi 313/346 R Primary Examiner-Ronald L. Wibert Assistant ExaminerPaul A. Sacher Attorney-Karl Limbach et al.

[57] ABSTRACT A cathode is disclosed utilizing an emission material of barium oxide, calcium oxide and an oxide selected from the group consisting of cobalt oxide, manganese oxide and molybdenum oxide.

9 Claims, 3 Drawing Figures THERMIONIC CATHODE The present invention is directed in general to cathodes and, more particularly, to thermionic dispenser cathodes.

Over the years various different cathode structures have been proposed with selected different materials to achieve high current densities, low evaporation and long life. Cathodes have been made in many forms. In one form, the emission material is sprayed or painted on the surface of a support member, typically nickel, tungsten or molybdenum. Ribbons, wires or screens have been added to the surface to aid in holding the emission material on the surface. Additionally, porous body structures have been utilized to slowly dispense the emission material through the pores to the emission surface. U. S. Pat. No. 2,543,728 to Lemmens et al. describes the use of a cavity to contain the emission material and a porous body portion providing communication between the cavity and the electron emission surface for migration of the emission material. U. S. Pat. No. 2,700,000 to Levi et al. describes a cathode wherein the emission material is melted into the pores of the porous-sintered body.

Barium oxide (BaO) has been the principal preferred emission material for many cathodes. However, barium oxide is extremely hygroscopic and in normal atmosphere readily converts to barium hydroxide. Consequently, many cathodes include barium oxide in a composition with another material whereby the barium oxide and/or free barium can be released for desired electron emission. Barium carbonate has frequently been used as one composition wherein the carbonate will convert to barium oxide at elevated temperatures.

In thermionic dispenser cathodes operating at higher temperatures and utilizing refractory metal bodies such as tungsten, molybdenum and the like as the poroussintered main body portion of the cathode, barium carbonate reacts with tungsten in a manner unproductive of free barium. Consequently, a number of alkaline earth metal compositions are suggested wherein barium oxide will be held in a stable form. Materials suggested in U. S. Pat. No. 2,700,1l8 to Hughes et al. are alkaline earth metal silicates, aluminates, thorates, berylliates, and borates. Of these materials, the barium aluminates were preferred and have been used most extensively.

Since it was known, as in Thermionic Emission from the BaO-CaO System by L. E. Grey, NATURE, Volume 165, pp. 773-774, May 13, 1950, that enhanced emission can be achieved by the substitution of calcium oxide for part of the barium oxide, barium calcium aluminate dispenser cathodes have been used. U. S. Pat. No. 3,076,916 to Koppius describes one such dispenser cathode having a porous-sintered tungsten body with an impregnate material formed of barium oxide, calcium oxide and aluminum oxide in the mole ratio of 4:1:1, respectively. U. S. Pat. No. 3,201,639 describes a similar thermionic dispenser cathode wherein the impregnate material is barium oxide, calcium oxide and aluminum oxide in the mole ratio of 5:312, respectively.

Broadly stated, the present invention is directed to a cathode having a metallic body and an emission material comprising a mixture of barium oxide, calcium oxide and an oxide selected from the group consisting of cobalt oxide, manganese oxide and molybdenum ox ide.

It has been found that cathodes having the preferred compositions of the present invention produce enhanced emission over cathodes similarly constructed but having the most popular prior art emission material made up of barium oxide, calcium oxide and aluminum oxide. More importantly, the cathodes of the present invention can produce the same emission as the most popular prior art cathodes but up to about 50 to 200C. cooler. This permits a lower heater wire temperature and faster tube warm-up time to achieve emission and requires less heater power.

IN THE DRAWINGS:

FIG. 1 is a side sectional view of a cathode in accordance with one embodiment of the present invention.

FIG. 2 is a graph of emission in milliamps plotted versus life for cathodes in accordance with the present invention as contrasted with cathodes in accordance with the prior art.

FIG. 3 is a graph of current in milliamps plotted versus cathode temperature for the cathodes in accordance with the present invention as contrasted with cathodes in accordance with the prior art.

- Referring now to FIG. 1 there is shown one embodiment of the present invention. In this embodiment of the present invention the cathode 10 includes a porous metallic body 11, of a refractory material, such as tungsten, provided with an emission material 12 substantially uniformly dispersed throughout the pores of the body for migration through the pores to an emission surface 13. The emission material 12 can be placed in the pores by mixing, pressing and sintering material 12 with metal particles such as tungsten particles when the body is of tungsten. Alternately, the cathode 10 can be formed by first forming the porous sintered body 11 by pressing and sintering tungsten particles and then melting the emission material into the pores of the body. An example of such a cathode is described below.

The body 11 is supported for use in a-vacuum tube by means of a support cylinder 14, such as molybdenum. A heater filament 15 such as a metal wire wound in a spiral is positioned within the cylinder 14 behind the body 11 for heating the body 11 and emission material 12 for thermionic emission of electrons from the emission surface 13 when the cathode is positioned within a vacuum envelope (not shown). Cathodes in accordance with this invention are suitable for use in most vacuum tubes or gas tubes. The cathode can be used as a cold cathode" wherein the heater 14 is utilized only to initiate emission or dispensed with entirely and emission derived in other ways such as by electron bombardment to produce secondary emission.

The heater filament 15 is placed in location within the cylinder 14 and potted" in placed with an electrical insulating material'lfi such as alumina ceramic.

The emission material 12 is a mixture of barium oxide, and an oxide selected from the group consisting of cobalt oxide, manganese oxide and molybdenum oxide, and another alkaline earth metal oxide, other than bar ium oxide, preferably calcium oxide. I prefer as an emission material one comprising barium oxide, calcium oxide and either cobalt oxide, manganese oxide or molybdenum oxide. Specific compositions of the emission material in accordance with the present invention are set forth in greater detail below and with reference to the method of formation of these materials.

l have discovered that a cathode made generally in accordance with the present invention but utilizing only barium oxide and either cobalt oxide, manganese oxide or molybdenum oxide does not produce satisfactory emission while an identical cathode body impregnated with an emission material of barium oxide, calcium oxide and either cobalt oxide, manganese oxide or molybdenum oxide produces satisfactory emission at a lower temperature than the popular barium oxide, calcium oxide and aluminum oxide type cathodes.

l have discovered that cathodes with emission materials formed of 4 moles of barium oxide, 1 mole of calcium oxide and 1 mole of either cobalt oxide, manganese oxide or molybdenum oxide as well as moles of barium oxide, 3 moles of calcium oxide and 2 moles of either cobalt oxide, manganese oxide or molybdenum oxide produce greater emission than similar cathodes with emission materials of barium oxide, calcium oxide and aluminum oxide in these same mole ratios.

FIGS. 2 and 3 show graphs of emission versus life and emission versus operating temperature for cathodes in accordance with the present invention designated C, Mn and Mo where the oxide other than barium oxide and calcium is cobalt oxide, manganese oxide or molybdenum oxide, respectively, as contrasted with the more popular barium oxide, calcium oxide and aluminum oxide cathodes designated type B. The 1 (i.e., B-l, C-l, Mn-l and Mo-l) refer to cathodes where the respective materials are in the mole ratio of 4:121 except for C which is 4212.1. As shown in the drawing, good emission is achieved when the proportion of cobalt oxide is as low as 0.1 mole.

While it is believed that the invention has been described thus far in sufficient detail to enable one skilled in the art to manufacture cathodes in accordance with the present invention, a detailed operative example of a cathode of the type illustrated in FIG. 1 will be given.

First of all, the body 11 is formed by pressing tungsten powder, approximately 5 microns average diameter, to form an ingot which is heated in hydrogen at about 2350C. for approximately minutes to form a porous-sintered body having a density in the range of 78-84 percent. Next the porous body is impregnated with a filler material such as a metal or plastic and the filled ingot machined to form individual buttons or bodies having the dimensions of the desired cathodes. In the case of metal, such as copper, the filler material can be melted into the pores, while in the case of a plastic such as methyl methacrylate, the porous body can be impregnated with the liquid plastic which then sets up and hardens in the pores. After machining, the filler material is removed by heating to elevated temperature.

For the embodiment having an emission material of barium oxide, calcium oxide and cobalt oxide in the mole ratio 4:1:1, 4 moles barium carbonate, 1 mole calcium carbonate and 1 mole cobalt oxide are heated to a temperature of about l,000C. in an air atmosphere. At this elevated temperature, the carbonates convert to the respective oxides and these oxides form compounds. The resultant material is allowed to cool.

' For impregnating the emission material into the oathode body the cooled, hardened mass is pulverized to form a powder and heated in a hydrogen atmosphere to a temperature of about 1550C. and brought to contact with the previously machined cathode body whereby the emission material will migrate into the pores of the body by capillary attraction. Excess emission material is removed from the emission surface.

The heater may be potted (before impregnation) within the cylindrical support behind the cathode body by surrounding the heater with powdered alumina ceramic and heating the assembly to a temperature of about l900C. to sinter the alumina. The process is the same for making other cathodes in accordance with this invention by substituting manganese oxide or molybdenum oxide where cobalt oxide is indicated above. While calcium oxide is preferred as the other alkaline earth metal to be utilized with barium oxide and the other oxide, other alkaline earth metal oxides such as strontium oxide and magnesium oxide can be used.

The emission material can be applied to other surfaces such as by evaporation onto a metal surface from which emission is to be drawn.

Additionally the cathode can be made in the form of a directly heated cathode body as described in U. S. Pat. No. 3,514,661 to R. T. Reaves assigned to the assignee of this invention.

What is claimed is:

1. A cathode comprising:

a metallic body structure of a high melting point metal and an emission material supported by said body structure,

said emission material comprising a mixture of barium oxide, calcium oxide and an oxide selected from the group consisting of cobalt oxide, manganese oxide and molybdenum oxide.

2. The cathode of claim 1 wherein said oxide other than barium oxide and calcium oxide is cobalt oxide.

3. The cathode of claim 1 wherein said oxide other than barium oxide and calcium oxide is manganese oxide.

4. The cathode of claim 1 wherein said oxide other than barium oxide and calcium oxide is molybdenum oxide.

5. A dispenser cathode comprising:

a refractory metal body having an electron emission surface and a porous portion for migration of an emission material to said surface and emission material supported by and in contact with said body for migration through said porous portion to said surface,

said emission material comprising a mixture of barium oxide, a second alkaline earthmetal oxide other than barium oxide and an oxide selected from the group consisting of cobalt oxide, manganese oxide and molybdenum oxide.

6. The dispenser cathode in accordance with claim 5 wherein said second alkaline earth metal oxide is calcium oxide.

7. The dispenser cathode in accordance with claim 6 wherein said emission material comprises barium oxide, calcium oxide and cobalt oxide.

8. The dispenser cathode in accordance with claim 6 wherein said emission material comprises barium oxide, calcium oxide and manganese oxide.

9. The dispenser cathode in accordance with claim 6 wherein said emission material comprises barium oxide, calcium oxide and molybdenum oxide.

# t i t l 

2. The cathode of claim 1 wherein said oxide other than barium oxide and calcium oxide is cobalt oxide.
 3. The cathode of claim 1 wherein said oxide other than barium oxide and calcium oxide is manganese oxide.
 4. The cathode of claim 1 wherein said oxide other than barium oxide and calcium oxide is molybdenum oxide.
 5. A dispenser cathode comprising: a refractory metal body having an electron emission surface and a porous portion for migration of an emission material to said surface and emission material supported by and in contact with said body for migration through said porous portion to said surface, said emission material comprising a mixture of barium oxide, a second alkaline earth metal oxide other than barium oxide and an oxide selected from the group consisting of cobalt oxide, manganese oxide and molybdenum oxide.
 6. The dispenser cathode in accordance with claim 5 wherein said second alkaline earth metal oxide is calcium oxide.
 7. The dispenser cathode in accordance with claim 6 wherein said emission material comprises barium oxide, calcium oxide and cobalt oxide.
 8. The dispenser cathode in accordance with claim 6 wherein said emission material comprises barium oxide, calcium oxide and manganese oxide.
 9. The dispenser cathode in accordance with claim 6 wherein said emission material comprises barium oxide, calcium oxide and molybdenum oxide. 